1. Field
The present invention relates to data communication. More particularly, the present invention relates to a novel and improved interleaver for storing intermediate results for a Turbo decoder.
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other multiple access techniques. A CDMA system provides certain advantages over other types of system, including increased system capacity.
A CDMA system may be designed to conform to one or more CDMA standards such as the IS-95, cdma2000, and W-CDMA standards, which are known in the art. Each standard specifically defines the processing of data prior to transmission over the forward and reverse links. For example, speech information may be coded at a particular data rate, formatted into a defined frame format, and processed (e.g., encoded for error correction and/or detection, interleaved, and so on) in accordance with a particular processing scheme. The frame formats and processing defined by a particular standard (e.g., cdma2000 standard) are likely to be different from those of other standards (e.g., W-CDMA standard).
The W-CDMA standard employs a parallel concatenated convolutional encoder (often referred to as a Turbo encoder), which may be selected for encoding a code segment (i.e., a data packet) prior to transmission. The Turbo encoder employs two constituent encoders operated in parallel and in combination with a code interleaver. The code interleaver shuffles (i.e., interleaves) the information bits in the packet in accordance with a specifically defined interleaving scheme. One encoder encodes the information bits in the packet to generate a first sequence of parity bits, and the other encoder encodes the shuffled information bits to generate a second sequence of parity bits. The information bits and all or some of the parity bits in the first and second sequences are transmitted.
A complementary (and computationally intensive) Turbo decoding is performed at a receiver unit. For each Turbo encoded packet, the received bits are initially stored to a buffer. The information and parity bits for the first encoder are then retrieved from the buffer and decoded based on the first constituent code to provide “extrinsic” information indicative of adjustments in the confidence in the detected values for the information bits. Intermediate results that include the extrinsic information from the first decoder are then stored to a storage unit in an interleaved order matching the code interleaving used at the transmitter unit.
The intermediate results and the parity bits from the second encoder are then retrieved from their respective sources and decoded based on the second constituent code to provide extrinsic information indicative of further adjustments in the confidence in the detected values for the information bits. Intermediate results that comprise the extrinsic information from the second decoder are then stored to the storage unit in a deinterleaved order complementary to the code interleaving used at the transmitter unit. The intermediate results are used by the next iteration of the first constituent decoder. The decoding by the first and second constituent decoders is iterated a number of times to yield the final results.
For each information bit to be decoded, the storage unit is accessed to retrieve intermediate result generated for this bit by a prior decoding (if any). The intermediate result generated for each decoded bit is also stored back to the storage unit for use in a subsequent decoding. The storage unit is thus continually accessed as bits in the packet are decoded. For each memory access, the proper address needs to be generated such that the proper intermediate result is retrieved from the storage unit (for a read) or stored to the proper location (for a write).
As can be seen, efficient generation of addresses for memory accesses is highly desirable for efficient Turbo decoding, especially in light of a complicated interleaving scheme defined by the W-CDMA standard.